1. Field of the Invention
The present invention relates to a filter circuit that extracts a signal component corresponding to a desired channel from a frequency signal, and a receiving apparatus that includes the filter circuit.
2. Background Art
A superheterodyne receiving apparatus which once converts received radio waves into an intermediate frequency band signal is currently known. A problem with the use of the superheterodyne method is an image signal. To eliminate the image signal, a receiving apparatus using a complex band-pass filter as a filter in an intermediate frequency stage is known (for example, see Japanese Patent Application Laid-Open No. 2004-515104). A complex band-pass filter inputs intermediate frequency signals having a phase difference of 90° therebetween as a Q signal and an I signal, and passes desired waves and attenuates interfering waves with a desired band-pass characteristic.
For favorable reception performance, the complex band-pass filter desirably has a steep attenuation gradient with which to attenuate image waves or signals of unnecessary bands such as adjoining channels. To steepen the attenuation gradient, the filter order needs to be increased, with an increase in the circuit scale accordingly. Since the complex band-pass filter includes two systems of filters corresponding to the Q and I signals, respectively (for example see Japanese Patent Application Laid-Open No. 2008-205962), the amount of increase in the circuit scale due to the increased order is large.